The long-term objectives of this application are to obtain a detailed understanding of the structure, mechanisms of action, and regulation of the human DNA methyltransferase(s) (E.C.2.1.1.37). Available evidence strongly suggests that the postsynthetic modification of DNA catalyzed by this enzyme has a role in cellular differentiation and somatic inheritance. DNA methylation may also be involved in ectopic protein production, tumor initiation, and tumor progression. We propose to prepare and study milligram quantitites of the homogeneous enzyme from human placenta. Experiments designed to study the enzyme structure include the determination of sub-unit stoichiometry and molecular weight, and a partial determination of amino acid sequence. The purified enzyme will be used to construct and optimize an in vitro DNA methylation system. This system and antisera to the purified enzyme will be used in experiments to identify factors that may promote or inhibit DNA methylation during cell proliferation, and factors that may be responsible for the hypomethylation of DNA observed in certain transformed cell lines. The amino acid sequence information will be used to synthesize oligonucleotides for use as hybridization probes to aid in the isolation of a cDNA clone for the methyltransferase. The cDNA clone(s) thus isolated will be used to determine the genomic organization and the transcriptional regulation of the methyltransferase gene(s).